Refractory foam

ABSTRACT

A porous refractory in the K2O—SiO2—B2O3 system is formed by chemical direct foaming by heating to over 600° C., resulting in adherent black or white foam. The foam can function as highly porous thermal insulation, a high or low thermal emissivity surface, as a sealant for deteriorated refractory surfaces, as a filler for pockmarks/holes/gaps or as a bonding agent for parts with large gaps between them.

FIELD OF THE INVENTION

The invention disclosed herein relates generally to the field ofrefractory coatings and more specifically to a porous refractory foamformed from a liquid coating on heating.

BACKGROUND OF THE INVENTION

The parent applications referred to above define a tough, dense, hard,corrosion resistant, very flexible coating for ferrous metals found inthe system R₂O—SiO₂—B₂O₃ using K₂O, Na₂O or Li₂O whereby the coating ismade very corrosion resistant to aluminum by adding boron nitride (h-BN,hexagonal boron nitride) into the composition. All of thecharacteristics of these coatings as described therein are incorporatedherein by reference to the entire disclosure thereof. The systemsdescribed therein are capable of further uses and new discoveries withinthose systems.

SUMMARY OF THE INVENTION

It is an object of this invention to find further uses of compositionswithin the system R₂O—SiO₂—B₂O₃ as defined in the parent applications.

This object as well as other objects are accomplished utilizing therefractory from the K₂O—SiO2-B₂O₃ system which upon coating and firingproduces a highly porous layer formed from the foaming of thatcomposition upon heating.

DETAILED DESCRIPTION

According to this invention it has been found that upon coating andfiring to a temperature in excess of 600° C., an expanded foam coatinghaving a greater thickness than the original unfired coating results dueto the formation of foam therein. The foam, formed in situ by chemicaldirect foaming, provides a rigid, adherent, thermally insulating layer.The foam, once applied and formed, can function as highly porous thermalinsulation or as a high or low thermal emissivity surface, being eitherblack or white visually depending on starting liquid composition. Thisfoam can also be useful for sealing cracks in refractory materials,having gaps; deteriorated surfaces of refractory materials such asfilling pock marks, holes or gaps; or to bond parts having spaces therebetween. Compositions described herein adhere well to both ceramic andmetal surfaces.

Preferred compositional ranges for use in this invention are 2 to 30weight percent K₂O; 10 to 74 weight percent SiO₂; and between 23 to 79weight percent B₂O₃. The most preferred composition is 15 to 25 weightpercent K₂O; 35 to 50 weight percent SiO₂; and between 30 to 50 weightpercent B₂O₃. The useful temperature range can be extended withadditions of alumina, ceria, yttria, zirconia, hollow or dense glassmicrospheres, and other refractory materials in the form of powders,beads or fibers.

Cracks or other deteriorated surfaces of refractory material are sealedby applying the composition like a mortar and heating after drying to atemperature above 600° C., preferably about 700° C. causing thecomposition to foam. Upon heating to about 700° C., a foam forms andexpands. The foam exhibits a porosity of 45 to 65%, generally averagingabout 56%.

A preferred composition is 17 weight percent K₂O, 35.6 weight percentSiO₂, 47.4 weight percent B₂O₃ formed into a paint with water and thenheated to form the expanded foam. The thickness of the coating applieddepends on the initial application (after drying) and can vary dependingon the desired thickness of the resulting foam. Applied thicknesses arecommonly in the range of ¼″ (6.35 mm) to ¾″ (19.05 mm) but can beslightly thinner or thicker. Foaming takes place after a one time heatto 700° C., whether the coating is thoroughly dried or not. Therefractory foam has variable porosity depending on the heatingconditions and is typically about 5 times the original thickness of thedried coating.

As shown in the examples, the final foam layer can be visually black orwhite depending on the starting composition ingredients. This has theadvantage of controlling the emissivity of the resulting foam which,when applied to the interior of a furnace, can increase uniformity anddecrease heat up time. This foam can expand and fill damaged areas ofceramic or metal surfaces, adhering well to all refractory surfaceswhether dense or porous.

Having generally described this invention, the following specificexamples are given as further disclosures of the benefits thereof.

The following raw materials were used in these examples:

-   -   Potassium tetraborate tetrahydrate powder (K₂B₄ O₇·4H₂O)    -   Ammonium pentaborate tetrahydrate powder (NH₄B₅O₈·4H₂O)    -   Colloidal silica aqueous dispersion with 50 weight percent SiO₂        in water (trade name    -   Bindzil 9950, now Levasil F09950).    -   Potassium hydroxide solution with 45 weight percent KOH in        water.    -   Potassium silicate powder (trade name Kasolv 16) with 32.5        weight percent K₂O, 52.8 weight percent SiO₂, and 14.5 weight        percent H₂O; 1.6 weight ratio of K₂O to SiO₂.        A mixture of these raw materials can yield the correct molar        ratios for the potassium borosilicate main formulation of 12.4        mole percent K₂O, 40.7 mole percent SiO₂, and 46.9 mole percent        B₂O₃—which expressed in weight percent is 17 weight percent K₂O,        35.6 weight percent SiO₂, 47.4 weight percent B₂O₃. Also, a        different mixture of these raw materials can yield the molar        percentages of another favorable composition, 16.4 mole percent        K₂O, 53.7 mole percent SiO₂, 29.9 mole percent B₂O₃—which        expressed in weight percent is 22.5 weight percent K₂O, 47.0        weight percent SiO₂, 30.5 weight percent B₂O₃.

Example I

A liquid coating composition was formed with 55.1 grams of potassiumtetraborate tetrahydrate, 34.6 grams of ammonium pentaboratetetrahydrate powder, 71.2 grams of colloidal silica, 32 grams of watercontaining 2 percent cellulosic suspension agent. In weight percent,this composition is 28.6% potassium tetraborate tetrahydrate, 17.9%ammonium pentaborate tetrahydrate, 36.9% colloidal silica aqueousdispersion, and 16.6% water that contained 2 weight percent cellulosicsuspender. Upon heating-firing, this composition leads to 12.4 molepercent K₂O, 40.9 mole percent SiO₂, and 46.7 mole percent B₂O₃, whichexpressed in weight percent is 17 weight percent K₂O, 35.6 weightpercent SiO₂, 47.4 weight percent B₂O₃. This mixture was applied toseveral substrates and then heated to 700° C. in air whereby it formed ablack foamy glassy material after approximately ½ hour. This formulationwas thus found to bond well to ceramic, ferrous metal and reinforcedfiberglass material substrates.

Example II

A liquid coating composition was formed with 66.7 grams of potassiumtetraborate tetrahydrate, 6.6 grams of potassium hydroxide solution,94.0 grams of colloidal silica aqueous dispersion. In weight percent,this composition is 39.9% potassium tetraborate tetrahydrate, 3.9%potassium hydroxide solution, 56.2% colloidal silicasolution/suspension. Upon heating-firing, this composition leads to 16.4mole percent K₂O, 53.7 mole percent SiO₂, and 29.9 mole percent B₂O₃,which expressed in weight percent is 22.5 weight percent K₂O, 47.0weight percent SiO₂, 30.5 weight percent B₂O₃. This mixture was appliedto several substrates and then heated to 700° C. in air whereby itformed a white foamy glassy material after approximately ½ hour. Thisformulation was thus found to bond well with to ceramic, ferrous metaland reinforced fiberglass material substrates.

Example III

A liquid coating composition was formed with 66.7 grams of potassiumtetraborate tetrahydrate, 7.7 grams of potassium silicate powder, 86.0grams of colloidal aqueous dispersion. In weight percent, thiscomposition is 41.6% potassium tetraborate tetrahydrate, 4.8% potassiumsilicate powder, 53.6% colloidal silica solution/suspension. Uponheating-firing, this composition leads to 16.4 mole percent K₂O, 53.7mole percent SiO₂, and 29.9 mole percent B₂O₃, which expressed in weightpercent is 22.5 weight percent K₂O, 47.0 weight percent SiO₂, 30.5weight percent B₂O₃. This mixture was applied to several substrates andthen heated to 700° C. in air whereby it formed a white foamy glassymaterial after approximately ½ hour. This formulation was thus found tobond well to ceramic, ferrous metal and reinforced fiberglass materialsubstrates.

Example IV

The composition of Example I was mixed, dried, and then calcined at 700°C. for 4 hours in air (whether in stainless steel or ceramic crucible).A black foamy glass material was formed. It was noted that this mixturedoes not need to be dried or calcined before firing to form the foam.The foam formed after 30 minutes at 700° C.

This composition adheres to many ceramics, including fused silica andreinforced fiberglass material, and metals such as stainless steel. Thefoam does not adhere to BN parts or BN coatings. If a barrier coating isneeded, BN was found to work well.

The volumetric expansion of the foam was determined using theArchimedes' water displacement method. A known volume of the compositionof Example I was heated to 700° C. for 30 minutes. The resulting foamwas then submerged in water to determine the new volume. The volumetricexpansion of the composition was determined to be approximately 5.4times the original volume. The specific gravity of the foam wascalculated to be 0.28 g/cm³.

The foam was then crushed and uniaxially pressed in a 1-inch die with aforce of 20,000 pounds, and then re-fired to 700° C. for 30 minutes. Thespecific gravity of the resulting glass button was 0.63 g/cm³. Assuminga specific gravity of 0.28 g/cm³ as calculated from the results of thewater displacement test, this means the foam has approximately 56%porosity.

The effect of firing temperature was noted on tests performed onreinforced fiberglass:

-   -   200° C. will form a white crystalline material, which        delaminates from refractory ceramic; if applied on top of an        already-formed layer of foam, the white layer will stick at this        temperature.    -   300° C. will form a brown or rust-colored layer, which is        reasonably adherent to refractory ceramic.    -   Thus, adherence of the foam is established by heating the        applied layer to approximately 300° C. in air.

Rather than applying one thick layer, the foam can be gradually builtup, thus ensuring more robust mechanical properties, using the followingprocedure:

Apply and fire one layer-to approximately 300° C. or until the layerlooks burnt/brown; repeat as needed; fire the assembly to 700° C. for 30minutes to form a thick, robust foam layer.

Once formed, the foam can be reheated to at or near the foamingtemperature (650-800° C.) to form a hard, tough surface. Depending onthe additives or lack thereof, the foam can be used continuously fromapproximately 600 to 800° C. if the porous structure is to bemaintained. Additives such as alumina fibers and hollow glass sphereswere found to increase the strength of the foam and reduce the tendencyof the foam to collapse at elevated temperatures.

Example V

The effects of various additives were tested on the samecomposition—i.e., Example 1. It was observed that certain additives, inthe range of 2 to 30 weight percent addition, increased the strength ofthe foam after firing to 700° C. for 30 minutes. The same additives alsotended to decrease the volumetric expansion of the composition attemperature. Additions of alumina powder, alumina bubble, aluminafibers, mullite, perlite, vermiculite, zirconia bubble, hollowborosilicate microspheres and bamboo fiber were observed to impart anincrease in the mechanical strength of the foam. Additions of zinc oxideand certain compositions of hollow borosilicate microspheres lowered themelting temperature of the composition, such that the foam could form attemperatures less than 700° C. Additives such as cerium oxide, yttriumoxide and calcium borate had no observable effect on the foamingcapability or the strength of the foam. Those skilled in the art canutilize the addition of a myriad high temperature materials beyond thosementioned here to modify the chemistry of the foam.

Example VI

With the composition of Example 1, the effects of various additives weretested up to 1000° C. It was observed that most materials added to theoriginal composition did not allow the foam to maintain its porosityabove 800° C. in air. In those cases, the structure of the foamcollapsed, forming a tough, rigid, electrically insulating glass ceramiclayer. However, the addition of five weight percent alumina fiber (tradename Saffil HA) and fifteen percent hollow borosilicate spheres (tradename Sphericel 34P30 and Sphericel 60P18) did prevent slumping of thefoam up to 800° C. Specifically, adding 15 weight percent of Sphericel60P18 hollow glass beads with mean particle size of 17 micrometers ledto no evidence of flowing or collapsing of the foam at 800° C. Above800° C., the structure of the foam is molten and it begins to flow andcollapse.

Example VII

Multiple layers of the composition of Example 1 were applied and driedat 300° C. The additional coatings were applied at room temperatureafter each heating and each dried at 300° C. These multiple coatingswere then heated to approximately 700° C. for H hour which led to ablack refractory foam. Foam formed in this way will be more mechanicallyrobust than a single thick layer of the composition that is applied andfired to 600-700° C.

Example VIII

The-composition of Example 1 was coated onto fused silica and stainlesssteel parts and heated to 700° C. while still wet or damp or afterthorough drying. This resulted in a black glassy appearing hard toughadherent foam.

Example IX

This example utilized each independent constituent which were heated to700° C. for 4 hours. None of the individual constituents foamed. Thus,blends of the constituents leaving out one of them also did not foamwhen heated to 700° C. for 4 hours. It was thus found that all threecomponents shown in Examples I, II, and III must be present together forfoaming to occur.

The above examples indicate that heating to above 600° C. is required togenerate the refractory foam. This heating is often accomplished with atypical furnace and done in air atmosphere. The heating method shouldnot be limiting, since it is known that achieving the necessarytemperature can be done with many other methods, such as heat lamps,gas-fired torches, and even lasers or solar concentrators, as long asthe temperature of the composition reaches near “visual red heat” or thetemperature above 600° C. to cause the foaming action. Also, theatmosphere should not be limiting, since inert atmospheres such asargon, nitrogen, helium or even vacuum are not expected to affect theformation of this refractory foam. The time needed for the foam to form,determined by the composition, is generally given as 30 minutes to 4hours—which again should not be considering limiting for this refractoryfoam. It is recognized, as covered in the parent U.S. application Ser.No. 15/906,361, that the other alkali oxides, Na₂O and Li₂O, can beutilized alone or as mixtures instead of or with K₂O to producerefractory foam: thus, the use of R₂O with R being K, Na, Li iscontemplated and should not be considered limiting for this refractoryfoam. It is recognized that colloidal silica aqueous dispersions otherthan that mentioned above as 50 weight percent SiO₂ in water (trade nameBindzil 9950, now Levasil F09950) can be used such that the necessaryweight percent of SiO₂ can still be achieved: thus, varying the percentof colloidal SiO₂ in the colloidal silica aqueous dispersion should notbe considered limiting for this refractory foam. It is also recognized,as covered in the U.S. application Ser. No. 16/058,285 filed Aug. 8,2018, that the ingredients of this refractory foam can be produced in anon-aqueous solution/suspension/dispersion—so use of non-aqueoussolvents to create the foam should not be considered limiting for thisrefractory foam, nor should blends of water with non-aqueous solventsystems.

While the above detailed description was given with regards to specificcompositions, the scope of this invention should be defined only by thefollowing appended claims.

What is claimed:
 1. A process for forming a porous refractory coating ona ceramic or metal surface, or for repairing a refractory surface or forbonding to a refractory, ceramic, or metal surface of a part, bychemical direct foaming comprising the steps of: applying to saidsurface a liquid dispersion composition of 2 to 30 weight percent K₂O;10 to 74 weight percent SiO₂ and between 23 to 79 weight percent B₂O₃;heating said composition to a temperature above 600° C., thus causingsaid composition to foam and produce a highly porousthermally-insulating layer.
 2. The process according to claim 1, whereinsaid composition is 15 to 25 weight percent K₂O; 35 to 50 weight percentSiO₂ and between 30 to 50 weight percent B₂O₃.
 3. The process accordingto claim 1 wherein said composition comprises 17 wt. percent K₂O, 35.6wt. percent SiO₂ and 47.4 wt. percent B₂O₃.
 4. The process according toclaim 1, wherein said composition is produced from a mixture ofpotassium tetraborate tetrahydrate, ammonium pentaborate tetrahydrateand colloidal silica dispersion and leads to a black refractory foam. 5.The process according to claim 1, wherein said composition is producedfrom a mixture of potassium tetraborate tetrahydrate, potassiumhydroxide solution, and colloidal silica dispersion and leads to a whiterefractory foam.
 6. The process according to claim 1, wherein saidcomposition is produced from a mixture of potassium tetraboratetetrahydrate, potassium silicate powder, and colloidal silica dispersionand leads to a white refractory foam.
 7. The process according to claim1, wherein said composition further comprises addition of between 2 to30 weight percent of at least one of alumina, ceria, yttria, zirconia,borosilicate glass beads/spheres, mullite, perlite, vermiculite, andzinc oxide to tune the mechanical and thermal stability of the foam. 8.The process according to claim 1 wherein said step of heating is to 700°C.
 9. The process according to claim 1 wherein said composition aftersaid step of heating has a porosity of 45 to 65%.